An ignition system is a system for igniting a air-fuel mixture. Ignition systems are well known in the field of internal combustion engines such as those used in petrol (gasoline) engines used to power the majority of motor vehicles. Ignition system is divided into two electrical circuits - the primary and secondary circuits. The secondary circuit consists of the secondary windings in the coil, the high tension lead between the distributor and the coil (commonly called the coil wire) on external coil distributors, the distributor cap, the distributor rotor, the spark plug leads and the spark plugs.
Principle of operation of the secondary ignition circuit
The coil is the heart of the ignition system. Essentially, it is nothing more than a transformer which takes 12 volts from the battery and increases it to a point where it will fire the spark plug as much as 40,000 volts. The term "coil" is perhaps a misnomer since there are actually two coils of wire wound about an iron core. These coils are insulated from each other and the whole assembly is enclosed in an oil-filled case. The primary coil, which consists of relatively few turns of heavy wire, is connected to the two primary terminals located on top of the coil. The secondary coil consists of many turns of fine wire. It is connected to the high-tension connection on top of the coil.
Ignition systems can be divided into the following types:
- Distributor Ignition System
- Direct Ignition System (DI)
- Coil-on-Plug (COP) type – individual coil for each cylinder and the coil pack is mounted directly over the spark plugs.
- Individual coil for each cylinder with separate HT (high tension) leads.
- DIS-Wasted Spark Ignition - separate coil for each two cylinders.
Synchronous ignition with two secondary winding coil terminals.
The distributor ignition system is the most common ignition system for early model year vehicles. Distributor ignition systems use one coil that fires one spark plug at a time on the compression stroke only. Viewing the primary ignition pattern requires that you have to monitor the voltage signal on the negative side of the coil’s primary circuit and to identify the trigger cylinder by using the RPM probe.
The classical or conventional ignition system consists of the following components: ignition coil, distributor, spark plugs, high-voltage wires and some means of controlling the primary ignition circuit. The primary circuit of the ignition coil can contain: points, points controlling a transistor, the transistor being controlled by some other means (breaker less) or electronic ignition. In point-type ignition systems the current in the primary circuit is controlled by a mechanical switch (or breaker). The mechanical points may control a switching transistor which opens and closes the primary circuit of the ignition coil. In breaker less transistor and electronic ignition a Hall effect, VRS (Variable Reluctance Sensor) or an optical sensor may be used to control the switching transistor.
Current flows from the positive terminal of the battery, through the ignition switch and/or relay, through a fuse and on to the positive terminal of the ignition coil. The current returns to the battery through the negative terminal of the ignition coil, on through the switching device (points or a transistor) through the vehicle chassis, and to the negative terminal of the battery. While current is flowing in the primary circuit a magnetic field builds up in the ignition coil. Due to the inductance of the ignition coil it takes some time (1-6 mS, depending on design) for the primary current to reach its nominal value. When the primary current flow is interrupted, the magnetic field collapses rapidly (in about 20µS) and a high voltage is induced in the primary winding (CEMF Counter electro motive Force). This voltage is transformed in to a very high voltage in the secondary winding. The amplitude of this voltage depends on the turns ratio (commonly 100:1). A 300V primary voltage, therefore, will be 30 000V in the secondary winding. The voltage will only build until the break down voltage of the spark gap is reached - the firing voltage of the spark plug.
Direct Ignition System (DI)
COP systems use one individual coil for each spark plug. Each coil is located directly on top of its spark plug and does not use any external spark plug wires. Each coil pack also has an independent primary circuit which must be tested individually.
The individual ignition coil by one running cycle of the engine generates one ignition spark. Therefore, in individual ignition systems is required synchronization of coils work with position of a camshaft.
At submission of the voltage to the primary coil, the current starts to flow by a primary coil and because of that in the core of the coil changes the value of the magnetic flux. Change of the magnetic flux value in the core of the coil leads to occurrence of the voltage of positive polarity on a secondary coil. Because the speed of increasing of the current in the primary coil is slow, the voltage arising on a secondary coil is small – according 1…2 kV. But in the certain conditions the voltage value can be sufficient for untimely occurrence of the spark between electrodes of a spark plug and as consequence, too early ignition of the air/fuel mixture. In order to prevent possible damages of the engine due to untimely occurrence of the spark, formation of the spark between electrodes of a spark plug at submission of a voltage to a primary coil should be excluded. In the individual ignition systems, occurrence of this spark is prevented by means of built-in diode EFU to the ignition coil switched consistently in a circuit of a secondary coil.
At the moment of closing of the output ignition cascade, current in the primary circuit sharply interrupts, and the magnetic flux promptly decreases. This fast change of the magnetic flux value is causes to occurrence of the high voltage on a secondary coil of the ignition coil (under certain conditions, the voltage on a secondary coil of the ignition coil can achieve 40…50 kV). When this voltage achieves the value providing formation of the spark between electrodes of a spark plug, the compressed in the cylinder air/fuel mixture is ignited from the spark between electrodes of a spark plug.
In some systems coils are not located directly on top of each spark plug and external spark plug HT leads are used.
Each coil pack also has an independent primary circuit which must be tested individually.
DIS-Wasted Spark Ignition
DIS ignition systems use one coil for every two cylinders, also called “waste-spark” systems. A waste-spark system fires one coil for each pair of cylinders that are at top dead center (TDC) at the same time. These cylinder pairs are called “running mates.” One cylinder is at TDC on the compression stroke, while the other is at TDC on the exhaust stroke. The spark in the cylinder at TDC on the compression stroke ignites the air-fuel mixture to produce power. The spark in the cylinder at TDC on the exhaust stroke is “wasted,” hence the name “waste-spark.” Each waste-spark DIS coil is hooked in series with its two spark plugs. As the coil fires, secondary current creates a high voltage spark across the gaps of both plugs. One plug fires with the traditional forward polarity of an ignition system: negative (—) to positive (+) The other plug fires with opposite polarity: positive (+) to negative (—) Thus, one plug always fires with what has always been called “reversed polarity.” The voltage capacity of a DIS coil is high enough, however, to ensure that the available voltage is always high enough to fire the plug with reversed polarity when it’s on the compression stroke.
Elements of the Secondary Ignition Pattern
These parameters are practically the same for all types of ignition systems.
It is very important to understand each part of the ignition coil waveform and what went into making the amplitude or voltage change over time. Without an in-depth understanding of the waveform, you will not know what has failed. For a proper diagnosis of this waveform you must pay close attention even to the smallest changes.
- Angle of closed state of contacts - (dwell angle or dwell period)
This is the angle on which the crankshaft rotates from the beginning of accumulation of energy in the primary winding of the ignition coil until occurring of the spark in the spark plug.
In ignition systems with mechanical switch, these are the degrees to which the crankshaft is rotated from the moment of closing the contacts of the breaker until they open again.
In ignition systems without mechanical switch, this is the time during which the ECU allows to current to flow through the primary winding of ignition coil. The beginning of the current flow is determined from the opening electronic powerful switch and in the end of the current flow and hence the appearance of the spark is determined by the time of obstruction of electronic powerful switch.
Dwell time is the length of time the coil primary circuit is being completed, and current is flowing through it. The initial oscillations in the pattern are the result of the initial build-up of the magnetic field that is created around any conductor with current passing. As that magnetic field builds in strength, it causes a "Counter Electromotive Force" that opposes the current flow. This is why the pattern starts to take a slight upward slope.
This is angle on which the crankshaft is rotating at the moment when the spark arises until reaching the relevant cylinder at the top dead center. One of main tasks of any ignition system is to ensure optimum angle of advance in case of a spark. To ensure maximum power, the mixture must be ignited before the piston, which is in pumping cycle to reach its top dead center - so after reaching the top dead gases can have a maximum pressure and maximum useful work carried out during the working stroke of the piston. Also any ignition system provides interrelationship between the angle of the advance of spark, engine speed and engine load. When a spark uprise at a time that does not correspond to the optimum advance angle deteriorates the engine performance and increases fuel consumption.
At higher speeds, the speed of movement of the pistons is increased at this time to burn the mixture does not change - so the spark must occur earlier. Therefore the advance must be increased.
At the same speed of the crankshaft, the throttle position (throttle) may vary. This means that the cylinder will form a mixture of different composition and burning rate of the mixture depends on its composition. At fully open throttle (fully depressed accelerator pedal) the mixture burns faster and should be lit later - thus, when the engine load is increased, you must reduce the advance. Conversely, when the throttle is not tightly closed, the burning rate of the working mixture is less, so you need to increase the advance.
- Drilling voltage - Firing Line
This is the value of voltage in the secondary circuit at the time of appearance of the spark. In fact this is the maximum voltage in the secondary circuit. It directly depends on the distance between the electrodes of spark plugs and the mixture in the cylinders. A spark incurred at the time, which interrupts the current flow through the primary winding of the ignition coil. Typical value of this tension is between 7 kV and12 kV.
- Voltage combustion of the mixture - "spark Kv"
The point when the actual spark across the gap starts to take place. This part of the pattern is called "spark KV", or the energy required to actually initiate the spark and keep it going. Spark KV is affected by the actual resistance of the secondary circuit, from the ignition wire, through the plug, across the gap to ground. High spark Kv means higher than normal resistance, and lower spark Kv means lower than normal resistance. Tensions in the secondary circuit of ignition during the combustion of spark are usually between 1 kV-2 kV.
- Burn Time - spark line (also called “spark duration”)
Spark combustion length is normally between 1.5 mS to 2 mS.
The "spark line" is the actual time the spark is moving across the plug gap. Normally, this should be between 1.5 mS to 2.0 mS. Anything under 0.8 mS usually means that a misfire has occurred. It is affected by the circuit resistance, just like spark KV,
but the neat thing about the burn line is that it is a window into the combustion process.
All these parameters are shown in fig.1 below.
Fig.1 Secondary ignition waveform
Procedure to verify the reliability of the secondary ignition circuit
— Ohmmeter and voltmeter measurements —
- Measure the coil’s secondary winding resistance with ohmmeter. Normal resistance must be around 7000Ω.
- Switch ignition on but do not start the engine.
- Use a voltmeter to check whether battery voltage is applied to the coil’s positive terminal (usually “2”) and chassis ground.
— Oscilloscope measurements —
To perform a diagnosis of secondary voltage of all ignition systems, it is necessary to monitor the ignition coils secondary winding charge waveforms by clamping capacitive pick-up probes around each cylinder high tension lead.
To test all types of ignition systems, your oscilloscope must have features for motor tester (engine analyzer).
You can’t use an ordinary lab scope!
If you are using an ordinary lab scope, you can still perform a secondary ignition measurement but you’ll be observing only one cylinder waveform at a time. Then move the test probe to the next cylinder.
Distributor ignition system
- Connect a capacitive pick-up clamp to the coil wire, as close to the ignition coil as possible.
Connect the other capacitive pick-up clamp lead to the oscilloscope according to its instructions.
- Connect a 1st cylinder trigger pick-up clamp to the spark plug wire №1, as close to the spark plug as possible.
Connect the other cylinder trigger pick-up clamp lead to the oscilloscope according to its instructions.
- Start the engine and left idling.
- Watch the oscilloscope screen and compare it with the waveform in fig.2.
Normal operating parameters for an ignition system are as follows:
- Firing voltage - also known as break down voltage - on the average 4-18 kV;
- Spark voltage - also known as burn voltage 1-4 kV;
- Spark duration - also known as burn time 1-2 ms.
Note: The above values will change according to the air/fuel ratio and cylinder pressure.
DIS (Waste Spark)
To perform a diagnosis of secondary voltage of DIS ignition system, it is necessary to monitor the ignition coils secondary winding charge waveforms by inserting capacitive pickups to each cylinder HT cable.
If you are testing DIS – wasted spark and you have an access to the HT spark plugs leads, follow the specific instructions for your oscilloscope:
- Put a capacitive pick-up clamp on each cylinder and connect it according to the oscilloscope specific instructions.
- Start the engine and left it idling.
- Compare result with the waveform in fig.3.
Normal operating parameters for a DIS (Waste Spark) ignition system are as follows:
- firing voltage, also known as break down voltage – on the average 10…15 kV;
- spark voltage – also known as burn voltage 1…2 kV;
- spark duration – also known as burn time ~1,5 mS.
Direct Ignition (individual ignition)
If you are testing Direct Ignition (DI) system and you have an access to the HT spark plugs leads, follow the specific instructions for your oscilloscope.
The basic tested parameters at diagnostics of individual ignition are:
- presence of damped oscillations in the end of a site of burning of a spark between electrodes of a spark plug;
- duration of the period of accumulation of energy in a magnetic field of the individual ignition coil (usually 1,5…5,0 mS depending on the design of the coil);
- duration of burning of a spark between electrodes of a spark plug (usually 1,5…2,5 mS depending on the design of the coil). It is necessary to consider, that if the duration of spark burning between electrodes of a spark plug on any mode of engine work will be less than 0,5 mS because of the fail ignition coil, than the spark between electrodes of a spark plug will arise, but air/fuel mixture from such spark will not be ignited. If you are testing a COP system and you don’t have an access to the HV cylinder spark plugs wires, there are two ways to test the secondary ignition system:
- with COP pick-up sensor
Fig.4 COP sensor
The COP sensor can be used as an additional pick-up probe to any automobile oscilloscope. The COP sensor is used to test the efficiency of a COP ignition system, it is used to determine whether the COP system makes the ignition sparks it’s supposed to and their duration. The COP sensor transforms the signal induced in it by the ignition coil’s high-voltage chain into a voltage pulse, which are shown on the oscilloscope’s screen. These values are in direct relation to the voltaic arc produced by the spark plug. Because of the variety of ignition coils with different constructions, it should be noted that the COP sensor cannot determine the exact value of the voltage where the ignition spark appears, but it can be used for a comparative analysis between the different cylinders in order to determine which cylinder is not working properly.
With an ordinary lab scope you can use only one COP sensor to test each cylinder separately (one by one).
But if you want to test all cylinders simultaneously, you must use an engine analyzer (special oscilloscope with motor tester features). Also, the oscilloscope must have at least 4 channels.
Note: In this case you can’t use an ordinary lab scope!
- Put a COP sensor on each cylinder and connect it according to the oscilloscope specific instructions.
Output voltage of the COP sensor is around 1-2 volts.
- Start the engine and left it idling.
- Compare result with the waveform in fig. 5.
Note: Secondary voltage can rise up to 40000V!
- with universal HT extension leads
Fig.6 Universal coil-on-plug extension lead
Universal coil-on-plug extension leads are designed to aid diagnostics on secondary ignition circuits by allowing an HT measurement to be made when there is no, or limited access, to any spark plug leads.
- Connect the universal coil-on-plug extension leads between the coil pack and the spark plugs for each cylinder.
- Than attach a capacitive secondary ignition pickup on each lead.
- Start the engine and left it idling.
- Compare result with the waveform in fig.5.
Note: Secondary voltage can rise up to 40000V!
— Typical reasons for malfunction of the secondary ignition circuit —
Windings of some compact individual ignition coils are executed so, that the secondary waveform of such coils little differs from the waveform shown above. The most essential difference is presence of damped oscillations after breakdown of a spark interval between electrodes of a spark plug.
Secondary waveform of the serviceable compact individual ignition coil,
received with help of the universal capacitive probe. Presence of damped oscillations after breakdown of a spark interval between electrodes of a spark plug (the site is noted by a symbol "2" in fig.6) is consequence of design features of the coil and not an attribute of malfunction.
Secondary waveform of the faulty compact individual ignition coil, received with help of the universal capacitive probe.
An attribute of malfunction is absence of damped oscillations after the ending of burning of a spark between electrodes of a spark plug (the site is noted by an arrow in fig.7).
• Bad HT lead(s).
• Bad isolation of the secondary coil (when COP system is present) - It is possible to find this malfunction by the the primary or the secondary waveform. Attribute of the breakdown between a coil isolation of the ignition coil is absence of damped oscillations in the finish of burning of the primary or the secondary waveform.